ERROR: Invalid conversion from ‘const char*’ to ‘char*’ debugging homework problem - c++

Question

I'm new to C++ and am having a hard time debugging this. Any idea why I am getting this error? The upper function is supposed to take in a pointer to a C-string as an argument, iterate through each character in the string and convert it to uppercase. Also, how can I get my function to return the string in uppercase?
#include <iostream>
#include <string>
#include <string.h>
using namespace std;
string upper(char* some_string){
for (int i=0; i < strlen(some_string); ++i){
toupper(some_string[i]);
cout << some_string[i];
}
return "Done"
}
int main(){
std::string word;
cout << "Enter a string: ";
cin >> word;
upper(word.c_str());
}

Answer 1

Make the argument const char* because the function doesn't modify the string.
Apply toupper() to what should be printed. (toupper() returns int, so the result should be casted to char to have it be printed as characters)
Add a semicolon after return "Done".
#include <iostream>
#include <string>
#include <string.h>
using namespace std;
string upper(const char* some_string){ // use const char*
for (int i=0; i < strlen(some_string); ++i){
cout << static_cast<char>(toupper(some_string[i])); // apply toupper() to what to print
}
return "Done"; // add a semicolon
}
int main(){
std::string word;
cout << "Enter a string: ";
cin >> word;
upper(word.c_str());
}